Polymers are frequently used for producing lithium ion accumulators, e.g. as polymer electrolyte or for producing electrodes, it is usual to use, for example, polyvinylidene fluoride for producing the electrodes.
For this purpose, polyvinylidene fluoride is usually dissolved in an organic solvent and a suspension is produced by addition of lithium storage materials and optionally conductive additives. The electrode is obtained by coating a support, for example a metallic support, with this suspension and subsequently removing the solvent, N-Alkylpyrrolidones, in particular N-methylpyrrolidone, have been found to be suitable solvents.
The solvent can be contaminated as a result of the above use. The coating process introduces impurities, and by-products of the solvent used are also formed as a result of the high temperatures required and/or the presence of oxidizing compounds, in the case of N-methylpyrrolidone (NMP), compounds of the above formulae I and II, where R is an H atom, are formed by undesirable secondary reactions. These compounds have a boiling point similar to that of NMP and are therefore very difficult to separate from NMP by distillation.
Only solvents having a high purity and freedom from water can be used for producing long-lived lithium ion accumulators. A solvent which has been used once can only be reused when it has been purified and once again meets the demanding requirements in respect of purity and freedom from water.
JP 11071346 and JP 10310795 disclose purifying NMP which has been used as solvent for polyvinylidene fluoride by addition of acidic compounds, in particular by contact with acidic solids.
WO 2010/057917 describes the purification of NMP which has been used as solvent for polyvinylidene fluoride and the production of lithium ion accumulators. The purification is effected by addition of activated carbon, and the process is therefore a purification by adsorption.
The addition of alkali metal hydroxides or alkaline earth metal hydroxides to NMP has also already been described. Thus, distilling NMP in the presence or alkali metal hydroxides or alkaline earth metal hydroxides in order to prevent the formation of peroxides and stabilize the NMP is known from JP 2004284958 and JP 2007099890.
U.S. Pat. No. 4,965,370, JP 2001354769 and JP 11349566 describe the purification of NMP which has been used in the preparation of polyarylene sulfide; impurities from the polyarylene sulfide process can in each case be removed by addition of alkali metal hydroxides or alkaline earth metal hydroxides and the NMP can be reused for this process.
JP 03014559 discloses the removal of N-methylsuccinimide and phenol from NMP by addition of metal hydroxides or metal carbonates. Hare too, the NMP concerned is from the polyarylene sulfide process.
WO 03/053924 describes a process for preparing NMP from gamma-butyrolactone and methylamine. Here, metal hydroxides are used to remove excess gamma-butyrolactone by salt formation.
It was an object of the present invention to provide a process for purifying previously used N-alkylpyrrolidone, in particular NMP, which allows the reuse of the N-alkylpyrrolidones for any applications. In particular for the production of lithium ion accumulators. The process should be very simple and effective. Impurities of the above formulae I and II which have been introduced or formed should ideally be removed completely by the process.
We have accordingly found the process defined above.